Introduce and apply clang-format to Spider storage engine

I'd like to use clang-format to make the style of the code consistent. Applying clang-format to the whole existing code could result in messing up the commit history, but this seems not to be a big problem because, good or bad, the commit history of the Spider have not much information. Of course, I will use a config that doesn't change the existing code much in order to keep the history as far as possible. The code format should be checked by CI finally.